The most common problem we have with helium dilution FRC tests are leaks. Although the system tubing and spirometer bell leak occasionally, we do have valve failures relatively frequently. Valve failures are usually obvious but they sometimes only fail partially so leak checks are regularly performed on these test systems. We can’t perform leak checks on patients except while they are being tested however, and patient leaks are far more common than system leaks.
A technician asked me to look at a patient’s helium dilution FRC test because it had an odd helium tracing. The technician was sure the patient had been leaking but the FRC from this test was was actually the lowest of three tests and they weren’t sure why that was the case.
Once I saw it I was immediately able to tell the technician that it wasn’t a leak and that it was probably okay to report the results. I was able to say this because when there is a leak during a helium FRC test the helium constantly decreases and never plateaus. The rate of decrease may change but the most pertinent point is that the helium concentration never plateaus and even more importantly, it never increases.
I’ve seen this particular type of helium tracing before but to be sure I could properly explain what caused it I downloaded a table of system readings from the test software and they verified that what I thought happened was probably correct.